The hypothesis tested in this study is that prospective investigation of affected kindreds with multiple endocrine neoplasia (MEN)-IIa, MEN-IIb or FMCT will allow early detection, syndrome delineation & improved survival. The poly-endocrine neoplasms of MEN-II occur in 2 different inherited forms each with age dependent biochemical & clinical penetrance: MEN type IIa (Sipple's Syndrome) & MEN type IIb (multimucosal neurome syndrome). Medullary cancer of the thyroid (MCT) is the most common index lesion of both types.